<PAGE>

                                                                 EXHIBIT 10.28

                               DISTRIBUTOR AGREEMENT

This agreement is entered into this 1st day of September, 1998 by and between
Alcide Corporation, a Delaware corporation whose offices are located at 8561
154th Ave. N.E., Redmond, Washington, U.S.A. (hereinafter "Supplier") and
Merial Societe Par Actions Simplifiee, whose offices are located at 29 Avenue
Tony Garnier, B.P. 7123, 69348 Lyon Cedex 07, FRANCE (hereinafter
"Distributor"), and supersedes all previous agreements between the parties.

NOW, THEREFORE, in consideration of the mutual covenants contained herein, the
parties agree as follows:

     1.   DEFINITIONS

          As used herein, the term

          1.1  "Contract Term" shall mean that period stated on Schedule A
attached hereto.

          1.2  "Products" shall mean Alcide-Registered Trademark- external udder
care products sold through veterinarians, including UDDERgold-Registered
Trademark- Germicidal Barrier Teat Dip.

          1.3  "Territory" shall be as defined as France.

     2.   APPOINTMENT OF DISTRIBUTOR

          2.1  Subject to the terms and conditions of this Agreement, Supplier
hereby appoints Distributor as exclusive distributor to the veterinary channel
for the Products for the Territory.  Distributor hereby accepts said appointment
and agrees to actively promote and sell the Products.

          2.2  In accepting this appointment, Distributor agrees that it and its
subsidiaries shall not, directly or indirectly, sell or distribute in the
Territory, or develop:

                    (a)  Any other external udder disinfectant product during
the Contract Term.

                    (b)  Any product containing acidified chlorite, chlorous
acid or chlorine dioxide as its active ingredients or degradents during the
Contract Term and for a period of two (2) years thereafter, assuming a
manufacturing agreement.

                                     1
<PAGE>

          2.3  Distributor may appoint agents, dealers or sales representatives
to act on Distributor's behalf for sales of the Products in the Territory,
provided that any compensation to such agents, dealers or representatives shall
be solely Distributor's responsibility.

          2.4  Subject to the terms and conditions of this Agreement,
Distributor is authorized to sell the Products purchased from Supplier in such
manner, at such prices and upon such terms as Distributor shall determine.
Distributor is an independent contractor, not an agent or representative of
Supplier.  Distributor shall not assume or create any obligation in the name of
Supplier or make any representation, warranty or guarantee on behalf of or in
the name of Supplier.

          2.5  Labeling of the Products shall be determined jointly by Supplier
and Distributor.

                    In any of the Distributor's activities relating to the
promotion and sale of the Products, Supplier's name and trademark shall always
be prominently displayed in order to protect Supplier's rights and goodwill in
the same.  Whenever Supplier's name and trademark are used in advertising and
promotional programs, Supplier retains the right to review and approve same.

          2.6  All registrations, trade names, trademarks and product names
under which the Products are sold shall be the property of Supplier.  In the
event any registrations (e.g., Product registrations) are taken or issued in the
name of Distributor, Distributor shall, upon request, but in no event later than
upon termination of this Agreement, transfer such registrations to Supplier or
Supplier's designee and provide any documents and assistance reasonably required
in connection therewith.  It is preferred that all product registrations are
transferred to Supplier, at Supplier's expense, upon signing this contract.

          2.7  Supplier and Distributor each represent and warrant to the other
that it is authorized to enter into and perform this Agreement and that this
Agreement does not and shall not conflict with any other agreements it may have.

     3.   TERMS AND CONDITIONS OF SALE

          3.1  All of Distributor's orders for the Products shall be subject to
the terms and conditions set forth in this Section 3 and in the attached
Schedule D which provides product pricing.  No additional or different terms set
forth in Distributor's or Supplier's purchase order, acknowledgment or other
forms or correspondence (other than an amendment to this Agreement pursuant to
Section 8.1 hereof) shall govern any sales of the Products by Supplier to
Distributor.

          3.2  Supplier shall be responsible for labeling, packing and shipping
all Products ordered in a form agreed upon between Supplier and Distributor as
being appropriate for the Territory and suitable for ready sale to the end user
in the Territory.  All deliveries shall be F.O.B. Manufacturing Plant location.

          3.3  Schedule A (attached) sets forth a firm commitment of products to
be purchased by Distributor from Supplier during each year of the contract.  In
the event that Distributor does not take delivery of the commitment, Distributor
will pay Supplier $1.56 for each liter not taken.

                                     2
<PAGE>

          Ninety (90) days prior to the start of the contract year, Distributor
will provide Supplier with a twelve (12) month forecast of anticipated product
purchases by month, of which the first three (3) months shall be a firm purchase
order.  The twelve (12) month forecast will be updated each ninety (90) days to
facilitate Supplier's planning.

          Monthly purchase orders will be issued by Distributor to Supplier
ninety (90) days in advance.

          3.4  Distributor shall make payments to Supplier within sixty (60)
days following invoice of products ordered by Distributor.  Invoice will not be
issued by Supplier until Product is manufactured and ready for shipment with
proper notification of availability provided to Distributor.

          3.5  Supplier provides the Limited Warranty as described in Schedule
C.

          3.6  Prices shown in Schedule D may be revised by Supplier annually at
the anniversary date of this Agreement.

          3.7  Supplier shall have Distributor named as an additional insured
under Supplier's Product Liability Insurance policy at all times during the term
of this Agreement.

     4.   PROMOTIONAL ACTIVITIES

          4.1  Distributor shall undertake such advertising and promotional
activity relating to Products as is deemed appropriate by Distributor to
actively promote sales.  Such advertising and promotional activity shall be
solely at Distributor's expense unless otherwise agreed to in writing by
Supplier.  All advertising and promotional materials developed by Distributor
shall be in accordance with descriptions of Products provided by Supplier and,
to the best of Distributor's knowledge, shall be accurate in all material
respects.  Upon request, Supplier shall have the right to review and approve all
advertising and promotional materials developed by Distributor.  Such approval
will not be unreasonably withheld and will automatically be given if Supplier
does not respond to the request within seven (7) working days.

          4.2  Distributor's marketing plans shall be provided to Supplier
annually on or before the start of each contract year.  A list of major
meetings, annual shows, seminars and training programs at which Supplier's
participation is desired shall be submitted ninety (90) days in advance by
Distributor.

          4.3  A tabulation of Distributor sales by Product and Territory shall
be provided by Distributor to Supplier at the end of each fiscal quarter.

     5.   TERM AND TERMINATION

          5.1  This Agreement may be terminated by either party, effective
immediately upon notice to the other, in the event the party to which such
notice is sent becomes the subject of any bankruptcy or insolvency proceedings.

                                     3
<PAGE>

          5.2  In any case where a party claims the other party is in breach of
the provisions of this Agreement (other than a failure to purchase at least the
goals set forth in Schedule A), the injured party shall give written notice of
the breach.  The party in breach must commence curing the breach within sixty
(60) days of receiving notice thereof.  If the breach is not cured within one
hundred and twenty (120) days, this Agreement shall be terminated.

          5.3  The provisions of Sections 2.2, 2.6, and 7 and any accrued
obligations shall survive termination of this Agreement.

          5.4  No later than six (6) months prior to expiration of this
Agreement, Distributor and Supplier shall meet to discuss their intentions
regarding a new or extended agreement.

     6.   APPLICABLE LAW.

          This Agreement shall be governed by and construed in accordance with
the laws of the State of New York, regardless of its or any other jurisdiction's
choice of law principles.  The 1980 U.N. Convention on Contracts for the
International Sale of Goods shall not apply to this Agreement.  Settlement of
disputes relating to this Agreement shall be resolved according to the Rules of
Arbitration of the International Chamber of Commerce.

     7.   CONFIDENTIAL INFORMATION

          (a)  Supplier and Distributor agree, with respect to any confidential
information received from the other and identified as confidential information,
that:

               (i)  the receiving party shall use reasonable care to
prevent disclosure of the confidential information to any third party without
the prior written consent of the disclosing party, and the degree of care taken
by the receiving party shall be at least as great as the degree of care which
the receiving party takes in protecting its own confidential information; and

               (ii) receiving party shall not use confidential information
disclosed by the other party for any commercial purpose other than pursuant to
this Agreement, or publish or disclose it to third persons without the prior
written consent of the disclosing party.

          (b)  Neither party shall have any obligation with respect to any
information disclosed by the other party:

               (i)  which is already in the possession of the receiving
party at the time of its receipt from the disclosing party;

               (ii) which the receiving party lawfully receives from another
person whose disclosure thereof to the receiving party does not violate any
rights of the disclosing party; or

                                     4
<PAGE>

               (iii) which is or becomes published or otherwise publicly
available through no act or omission of the receiving party.

          (c)  Upon expiration or termination of this Agreement, Distributor and
Supplier shall each, upon the written request of the other, return or destroy
all materials, copies thereof and extracts therefrom which include any
information designated as confidential by the other pursuant to Section 7.1.(a).
Each may, however, retain for legal archival purposes only, one (1) copy of all
such material.

          (d)  The provisions of this Section 7.1 shall survive termination of
this Agreement and remain in full force and effect for a period of three (3)
years as to any item of confidential information.

     8.   MISCELLANEOUS

          8.1  This Agreement constitutes the entire agreement between
Distributor and Supplier and may be amended only by a written document signed by
both parties hereto.

          8.2  All notices, requests or other communications under this
Agreement shall be given in the English language and will be deemed properly
given if in writing and delivered in person, sent via international courier
service or by confirmed facsimile transmission to the intended recipient at the
address specified below, or to such other address as a party may specify in
writing:

                    If to Supplier:     Alcide Corporation

                                        Attn.:  Joseph A. Sasenick
                                        8561 154th Avenue N.E.
                                        Redmond, WA  98052
                                        U.S.A.

                 If to Distributor:     Merial

                                        Attn.:  Daniel Gouffe
                                        29 Avenue Tony Garnier - BP 7123
                                        69348 Lyon Cedex 07
                                        France

                                     5
<PAGE>

          8.3  The failure by either party to enforce any term or provision of
this Agreement shall not constitute a waiver of the same.

          8.4  This Agreement may be executed in any number of counterparts,
each of which shall be deemed an original and one and the same document.

          8.5  The rights of Distributor hereunder shall not be assigned or
transferred, either voluntarily or by operation of law, without the prior
written consent of Supplier, nor shall the duties of Distributor hereunder be
delegated in whole or in part.  Any such assignment, transfer or delegation
shall be of no force or effect.  Any change in control of Distributor shall be
deemed an impermissible assignment and entitle Supplier to terminate this
Agreement.  This Agreement shall be binding upon and inure to the benefit of
Supplier, its successors and assigns.

          8.6  If any provision of this Agreement is or becomes invalid, illegal
or unenforceable, the remaining provisions shall remain in full force and
effect, and for the invalid, illegal or unenforceable provision shall be
substituted a valid, legal and enforceable provision which shall be as similar
as possible in economic and business objectives as intended by the parties.

          8.7  Distributor shall comply with all applicable laws and regulations
in performing under this Agreement.

          8.8  Neither party shall be responsible for non-performance or delay
in performance arising from force majeure except the term of this Agreement
shall not be extended as a consequence thereof.


                                     6
<PAGE>

IN WITNESS WHEREOF, the parties have executed this Agreement on the date first
above written.


                                   ALCIDE CORPORATION
                                   ("Supplier")


                                   By  /s/ John Richards       7/17/98
                                      ------------------------
                                   MERIAL
                                   ("Distributor")


                                   By  /s/ Daniel Gouffe
                                      ------------------------



                                        7
<PAGE>

                                     SCHEDULE A

                                   (1) Contract Term
                                   (2) Commitments to purchase product

(1)  The Contract Term shall be a three year period commencing June 1, 1998, and
     ending May 31, 2001 (Fiscal Year 1999 through Fiscal Year 2001).

(2)  Product to be purchased by Distributor from Supplier for the Territory
     during the twelve (12) month period after commencement of the Contract Term
     are as follows:


<TABLE>
<CAPTION>
               France Veterinary Channel          Quantity
                                                  --------
               <S>                          <C>
               Alcide Fiscal Year

               1999                         430,000 use dilution liters
                                               (110,000 quarterly)

               2000                         537,500 use dilution liters
                                               (135,000 quarterly)

               2001                         671,875 use dilution liters
                                               (170,000 quarterly)
</TABLE>

                                        8
<PAGE>

                                     SCHEDULE B

                                     Territory






                                       France



                                        9
<PAGE>

                                     SCHEDULE C



                                  Limited Warranty

Alcide Corporation warrants to all purchasers of this Product that it has been
manufactured in accordance with U.S. regulatory requirements, is free of defects
and is as described in all labeling affixed hereto.  Alcide's sole obligation
under this warranty and buyer's sole remedy for any defect or failure to meet
such requirements or labeling shall be limited to replacement without cost
(except all costs for shipping and handling which shall be Distributor's
responsibility) of any quantity of the Product sold.

THE WARRANTY PROVIDED HEREIN AND THE OBLIGATIONS AND LIABILITIES OF ALCIDE
CORPORATION HEREUNDER ARE EXCLUSIVE AND IN LIEU OF, AND BUYER HEREBY WAIVES ALL
OTHER REMEDIES, WARRANTIES, GUARANTIES OR LIABILITIES, EXPRESS OR IMPLIED,
ARISING BY LAW OR OTHERWISE (INCLUDING, WITHOUT LIMITATION, ANY IMPLIED
WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY AND ANY
REMEDIES OR LIABILITIES FOR LOST PROFITS OR CONSEQUENTIAL DAMAGES).  BUYER
ACKNOWLEDGES THAT HE IS NOT RELYING ON THE JUDGMENT OF ALCIDE CORPORATION TO
SELECT OR FURNISH COMPONENTS OR MATERIALS SUITABLE FOR ANY PARTICULAR PURPOSE
AND THAT ALCIDE CORPORATION MAKES NO WARRANTIES OTHER THAN ON THE FACE HEREOF.


                                     10
<PAGE>

                                     SCHEDULE D

1.   Full use dilution from Alcide (gallons).

<TABLE>
<CAPTION>
     PRODUCT        SIZE           PRICE
     -------        ----           -----
<S>                 <C>            <C>
     UDDERgold      1 gal.         $9.64
     UDDERgold      5 gal.         $8.80
     UDDERgold      15 gal.        $8.60
     UDDERgold      55 gal.        $8.20
</TABLE>

2.   Concentrate (gallons) -- UDDERgold 55 gallon -- $25.00 per gallon.

3.   If Coophavet manufactures (liters) -- 20 liters Coophavet cost plus $1.56
     per liter.


                                     11